1. Field of the Invention
This invention relates generally to a device and method for and the treatment of iron-contaminated fluid (e.g., mining-related discharge, groundwater, surface water and industrial waste streams) and, more particularly, to an apparatus and method for oxidizing and removing ferrous iron from iron-contaminated fluid, including mine drainage, and producing an effluent substantially free of iron.
2. Description of the Prior Art
Iron-contaminated water results from a variety of natural and anthropogenic processes with the latter typically involving mining and industrial processing. Ferrous iron is released from minerals (e.g., pyrite, siderite, and hematite) through dissolution and redox processes. Industrial processing typically involves formation of reduced iron (Fe0) into various metallic compounds, with waste streams or subsequent oxidation causing elevated ferrous iron levels.
The most common source of iron-contaminated water results from mineral extraction and can be produced from either surface or deep mining practices where iron sulfide minerals contained in the minerals and surrounding formations are oxidized. The chemistry of mine drainage will vary depending on overburden characteristics and mining and reclamation techniques. In the United States millions of gallons of mine drainage is produced daily from both active and abandoned mine sites. Treating mine drainage is an expensive endeavor involving land, construction, materials, operation, maintenance and chemical costs. Left untreated, mine drainage contaminate surface and groundwater causing impacts to their social, recreational and commercial uses.
Iron is removed from iron-contaminated waters employing chemical and passive treatment technologies. Current chemical treatment, more commonly used for industrial sources and active mines, requires continuous metering of caustic chemicals (e.g., quick lime, hydrated lime or soda ash) to raise the pH above 8 thereby increasing the rate of iron oxidation and precipitation as oxides (USEPA 1981). In addition to chemical additives, active treatment requires an assorted array of pumps, aeration equipment and multiple oxidation and settling basins. Iron oxide solids produced in chemical treatment are low density (1 to 4% solids) and highly contaminated with calcium, aluminum, manganese, and sodium deposits (Dempsey & Jeon 2001). The low-density solids slowly settle in large open water basins, which require frequent and costly maintenance to remove and dispose the accumulated solids.
Passive treatment systems rely on natural amelioration processes that do not require pumps or metered chemical additions. In general, mine drainage passes through open water ponds and/or aerobic wetlands where abiotic and biotic processes contribute to the oxidation and precipitation of iron (Hedin & Nairn 1993). Iron removal in passive treatment systems require much larger land areas (10 to 20 times greater) than chemical treatment, which can become excessive for high flow and/or high iron concentration mine drainage discharges. In addition, iron removal in passive systems can be problematic with performance varying with season, influent flow and iron concentration, and alkalinity in the mine drainage. Iron oxide solids produced by passive treatment systems have much higher sludge density (15-30%) than chemical treatment and are frequently less contaminated (Dempsey & Jeon, 2001). Reported iron oxide content in passive treatment solids varies from 50 to 90%.
AIS-treated waters produce a unique iron oxide sludge that (1) settles at a rate faster than either chemically or passively produced solids; (2) is a high-density sludge with solids of approximately 30%; and (3) is a high-purity sludge with iron oxide content exceeding 95%. The prior art does not address the unique solids content of AIS-treated fluids.
Ferrous iron oxidation is usually the limiting step in the iron removal from iron-contaminated mine drainage. Iron oxidation has been described to occur by two separate processes known as homogeneous oxidation, a solution oxidation process, and heterogeneous oxidation, a solid/solution interface oxidation process. Homogeneous oxidation involves soluble Fe2+, FeOH+, or Fe (OH)2° species in the presence of dissolved oxygen (Stumm & Morgan 1996). This oxidation is strongly dependent on pH with slow oxidation occurring at pH 6 and rapid oxidation occurring above pH 8. Heterogeneous oxidation involves sorbed ferrous iron on the surface of iron oxides in which the iron oxide acts as a catalyst (Dietz 2003 and Tamura & Nagayama 1976). At high suspended iron oxide concentrations, heterogeneous oxidation has been found to produce oxidation rates greater than 100 times the rates observed in passive treatment and comparable rates to chemical treatment (Dietz 2003, and Dietz & Dempsey 2001)). Heterogeneous ferrous iron oxidation (HeFIO) is described by the following model:
                    ∂                  [                      Fe            ⁡                          (              II              )                                ]                            ∂        t              =                  -                  (                                    k                              He                ⁢                                                                  ⁢                1                                      ×                          [              DO              ]                        ×                                          1                +                                  (                                                            [                                                                        Fe                          ⁡                                                      (                            II                            )                                                                          diss                                            ]                                        ×                                          K                      1                      app                                                        )                                                                              [                                      ≡                                          Fe                      ⁡                                              (                        III                        )                                                                              ]                                ×                                  Γ                  1                                ×                                                      {                                          H                      +                                        }                                    1                                                              )                    -              (                              k                          He              ⁢                                                          ⁢              2                                ×                      [            DO            ]                    ×                                    1              +                              (                                                      [                                                                  Fe                        ⁡                                                  (                          II                          )                                                                    diss                                        ]                                    ×                                      K                    2                    app                                                  )                                                                    [                                  ≡                                      Fe                    ⁡                                          (                      III                      )                                                                      ]                            ×                              Γ                2                            ×                                                {                                      H                    +                                    }                                2                                                    )                                ⁢                  pK                  x          ,                      T            ⁢                                                  ⁢            2                          app            =                        pK                      x            ,                          T              ⁢                                                          ⁢              1                                app                -                  (                                                    Δ                ⁢                                                                  ⁢                                  H                                      rxn                    ,                    x                                    0                                                            2.303                ×                R                                      ×                                                            T                  2                                -                                  T                  1                                                                              T                  2                                ×                                  T                  1                                                              )                                        ⁢                  p        ⁢                                  ⁢                  k                      Hex            ,                          T              ⁢                                                          ⁢              2                                          =                        p          ⁢                                          ⁢                      k                          Hex              ,                              T                ⁢                                                                  ⁢                1                                                    -                  (                                                    E                                  a                  ,                  x                                                            2.303                ×                R                                      ×                                                            T                  2                                -                                  T                  1                                                                              T                  2                                ×                                  T                  1                                                              )                    
Summary of parameters and constants in the ferrous iron sorptionheterogeneous ferrous iron oxidation (HetOX) models.Sub-Sub-ModelModelModelParameterDescription(x = 1)(x = 2)[Fe(II)]Ferrous Iron Concentration, molarvariesVaries∂[Fe(II)]/∂tFerrous Iron Oxidation Ratevariesvaries[DO]Dissolved Oxygen Concentration,variesvariesmolar[Fe(II)]dissDissolved Fraction of Ferrous Iron,variesvariesmolar[≡Fe(III)]Suspended AIS as Ferric IronvariesvariesConcentration, g/L{H+}Hydrogen Ion Activity, molarvariesvaries{H+} = 10−pHkHex (M−1s−1)Oxidation Rate Constant0.10538.0Ea,x (kJ/mol)Activation Energy of Oxidation60.760.7ReactionKxapp (Mx−1)Surface Complexation Constant10−1.26510−10.78Γx (mol/mol)Sorption Site Density0.00450.212ΔH0rxn,x (kJ/mol)Enthalpy of Sorption Reaction69.096.2{H+}Hydrogen Ion Coefficient12Coefficient (x)Homogeneous oxidation is by far the dominant process in both chemical and passive treatment, typically accounting for greater than 95% of the oxidation. This occurs because (1) chemical treatment occurs at high pH were homogeneous oxidation is by far the fastest oxidation either with or without suspended iron oxide solids; and (2) passive treatment is a non-mechanical approach that does not allow for the suspension of high concentrations of iron oxide (>200 mg/L) that would be needed to have heterogeneous oxidation dominate ferrous iron oxidation.
Alkalinity may need to be generated to complete the precipitation of oxidized ferrous iron where the source water alkalinity (mg/L as CaCO3) to iron (mg/L as Fe) ratio is less than about 1.7. The low pH (approximately 5 to 6) and/or high carbonic acid concentrations (PCO2 approximately 0.1 to 0.5) found in many iron-contaminated waters (i.e., mine drainage) results in the rapid dissolution of carbonate minerals (such as calcite), thereby producing alkalinity at concentrations higher than will typically occur in natural systems. A type of passive treatment, known as Anoxic Limestone Drains (ALD), has been found to produce alkalinity greater than 300 mg/L (Hedin et al 1994). Other research has found carbonate dissolution occurs rapidly until pH greater than 6 is achieved and the rate of dissolution is directly proportional to the surface area of the carbonate mineral present (Amrhein et al 1985; Pearson & McDonnell 1974). Testing done with a relatively unused material, pulverized limestone, in AIS treatment has been shown to adequately address the alkalinity issue due to rapid dissolution of the carbonate in the high ferrous oxidation reaction rate environment of AIS in combination with the complete mixing in the AIS reactor.
Therefore, it is an object of this invention to provide treatment processes and apparatus for oxidizing and removing ferrous iron from iron-contaminated mine waters at pH (less than 7) typically found in iron-contaminated waters.
Another object of this invention is to oxidize and remove ferrous iron from iron-contaminated waters by using the higher oxidation rates supported by heterogeneous oxidation through mechanical suspension of high particulate iron oxide concentrations (i.e., >200 mg/L) and providing a source of alkalinity where inadequate alkalinity is present to complete the oxidation and precipitation of iron.
It is also an object of this invention to develop a simple means of collecting and concentrating the iron oxides produced by the iron-contaminated liquid treatment processes and apparatuses.
Other objects will be readily apparent after reading the description and reviewing the figures described below.